Universal dimmer

ABSTRACT

Disclosed is a universal dimmer which detects the type of load it is connected to each time the universal dimmer is activated. Information relating to the type of load connected to the universal dimmer is therefore not required to be stored for future use. In one form, the universal dimmer begins each operation in the trailing edge mode and upon detection of an inductive load, switches to leading edge mode.

TECHNICAL FIELD

This invention relates to dimmer circuits and in particular, to dimmercircuits able to operate as both leading edge and trailing edge devices,otherwise known as “universal dimmers”.

PRIORITY

This specification claims priority from:

Australian Provisional Patent Application No. 2005906990 entitled “AUniversal Dimmer” and filed on 12 Dec. 2005; and Australian ProvisionalPatent Application No. 2005906949 entitled “Load detector for a Dimmer”and filed on 12 Dec. 2005.

The entire content of each of these applications is hereby incorporatedby reference.

BACKGROUND

Dimmer circuits are used to control the power provided to a load such asa light or electric motor from an alternating current (AC) power sourcesuch as mains. Such circuits often use a technique referred to as phasecontrolled dimming. This allows power provided to the load to becontrolled by varying the amount of time that a switch connecting theload to the power source is conducting during a given cycle.

For example, if voltage provided by the power source can be representedby a sine wave, then maximum power is provided to the load if the switchconnecting the load to the power source is on at all times. In this waythe, the total energy of the power source is transferred to the load. Ifthe switch is turned off for a portion of each cycle (both positive andnegative), then a proportional amount of the sine wave is effectivelyisolated from the load, thus reducing the average energy provided to theload. For example, if the switch is turned on and off half way througheach cycle, then only half of the power will be transferred to the load.Because these types of circuits are often used with resistive loads andnot inductive loads, the effect of repeatedly switching on and off powerwill not be noticeable as the resistive load has an inherent inertia toit. The overall effect will be, for example in the case of a light, asmooth dimming action resulting in the control of the luminosity of thelight. This technique will be well understood by the person skilled inthe art.

Modern dimming circuits generally operate in one of two ways—leadingedge or trailing edge.

In leading edge technology, the dimmer circuit “chops out” or blocksconduction of electricity by the load in the front part of each halfcycle (hence the term “leading edge”).

In trailing edge technology, the dimmer circuit “chops out” or blocksconduction of electricity by the load in the back part of each halfcycle.

FIG. 1A shows a representation of the function of a leading edge dimmer,while FIG. 1B shows the function of a trailing edge circuit.

In FIG. 1A, the shaded region of the sine wave, representing the appliedAC power to the load, indicates the part of the cycle during which thedimmer circuit allows electricity to reach the load. The blank region infront of the shaded region indicates the part of the cycle that has beenblocked by the dimmer circuit, preventing power from being applied tothe dimmer circuit.

In FIG. 1B, the reverse situation, for the trailing edge, isillustrated. In this case, the shaded region at the beginning of the ACcycle indicates the part of the cycle during which the dimmer circuitallows electricity to reach the load. The blank region after the shadedregion indicates the part of the cycle that has been blocked by thedimmer circuit, preventing power from being applied to the dimmercircuit.

Which of the two technologies used for a particular application dependsupon the type of the load. Inductive load types (such as iron core lowvoltage lighting transformers and small fan motors) are best suited toleading edge operating mode, where the established half-cycle loadcurrent is terminated when at substantially low levels, thus avoidingundesirable voltage spiking. Capacitive load types are best suited totrailing edge operating mode, where the start-of-half-cycle applied loadvoltage ramps up from zero at a relatively slow rate, thus avoidingundesirable current spiking.

In practice, it has been necessary to select the appropriate dimmer forthe appropriate load. This requires stocking multiples of each type ofdimmer and has the risk that the incorrect dimmer is connected to agiven load.

An improved form of dimmer circuit, known as “adaptive” or “universal”dimmers has been developed, which can function in either the leadingedge or trailing edge mode. This alleviates the need to have multiplesof each dimmer type to cater for different loads, and the installer doesnot have to be particularly concerned about the load type. Additionally,from the manufacturing standpoint, only one build type of dimmer isrequired.

Universal dimmer designs incorporate a means to initially determinewhich operating mode is suitable for the connected load, in addition tonon-volatile memory elements for the purpose of retaining the operatingmode thereafter.

A number of techniques are used in the determination of the appropriatesteady state dimmer operating mode, for the type of load to becontrolled. Typical existing universal dimmer designs operate on thebasis of once-off determination of “load category” at first time ofoperation, and thereafter effectively behave as a standard dimmer(either leading edge or trailing edge type). A representation of theselected operating mode is stored in non-volatile memory, which isavailable for retrieval each time the load is to be operated.

While an improvement over single mode dimmers, existing universaldimmers suffer from the disadvantage that once a mode of operation hasbeen determined, the device must retain information as to whichoperating mode has been selected, while the device is not in operation.

SUMMARY

According to a first aspect of the present invention, there is provideda universal dimmer for controlling power applied to a load, theuniversal dimmer comprising a load type detector which in use, detects aload type upon each activation of the universal dimmer.

In one form, the universal dimmer is adapted to operate in the trailingedge mode upon activation and the load type detector comprises aninductive load detector for detecting the presence of an inductive load,and the universal dimmer further comprises a mode control circuit tochange the operation of the universal dimmer to leading edge mode if aninductive load is detected by the inductive load detector.

In one form, the inductive load detector comprises:

-   -   a voltage ringing detector for detecting a voltage ringing        signal across the load; and    -   a signal generator for generating a signal indicating the        presence of an inductive load upon the voltage ringing detector        detecting said voltage ringing signal.

In one aspect, the voltage ringing detector comprises a peak detectorfor detecting a peak value of the voltage ringing signal and forgenerating a dc signal corresponding to the detected peak value.

In one form, the inductive load detector further comprises a dcaccumulator for accumulating the dc signal over time to provide anaccumulated dc signal.

In another form, the inductive load detector further comprises acomparator for comparing the accumulated dc signal with a referencevoltage and producing an output when the accumulated dc signal exceedsthe reference voltage.

In a further form, the inductive load detector further comprises aninductive load indicator for generating a signal indicating the presenceof the inductive load when the output of the comparator indicates theaccumulated dc signal exceeds the reference voltage.

In a further form, the inductive load detector further comprises avoltage spike detector for detecting the presence of a voltage spikeacross the load and for generating a signal indicating the presence ofthe voltage spike.

In one form of the universal dimmer, the mode control circuit, in use,changes the mode of operation of the dimmer circuit from trailing edgeto leading edge operation upon detection of the voltage ringing signal.

In another form, the mode changing circuit, in use, changes the mode ofoperation of the dimmer circuit from trailing edge to leading edgeoperation upon detection of the voltage spike.

According to a second aspect of the present invention, there is provideda method of operating a universal dimmer used to control power appliedto a load, the method comprising causing the universal dimmer to detecta type of the load to which it is connected upon each activation of theuniversal dimmer circuit.

In one form, the method further comprises causing the universal dimmerto operate in the trailing edge mode upon activation and to change tothe leading edge mode if an inductive load is detected.

In one form, the step of detecting the presence of the inductive loadcomprises:

-   -   detecting a voltage ringing signal across the load; and    -   generating a signal indicating the presence of an inductive load        upon detecting said voltage ringing signal.

In one form, the method further comprises detecting a peak value of thevoltage ringing signal and generating a dc signal corresponding to thedetected peak value.

In one form, the method further comprises only detecting the peak valueof the voltage ringing signal during a brief period at each half cycleof the voltage signal applied to the load so as to minimise the effectof electrical noise.

In one form, the method further comprises accumulating the dc signalover time to provide an accumulated dc signal.

In another form, the method further comprises comparing the accumulateddc signal with a reference voltage and producing an output when theaccumulated dc signal exceeds the reference voltage.

In another form, the method further comprises generating a signalindicating the presence of the inductive load when the accumulated dcsignal exceeds the reference voltage.

In a further form, the method further comprises detecting the presenceof a voltage spike across the load and generating a signal indicatingthe presence of the voltage spike.

In another form, the method further comprises changing a mode ofoperation of a dimmer circuit from trailing edge to leading edgeoperation upon detection of the voltage ringing signal.

In one form, the method further comprises changing a mode of operationof a dimmer circuit from trailing edge to leading edge operation upondetection of the voltage spike.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to thefollowing drawings in which:

FIG. 1A shows a representation of leading edge dimmer operation;

FIG. 1B shows a representation of trailing edge dimmer operation;

FIG. 2—shows a universal dimmer according to an aspect of the presentinvention in which the load type is detected upon each activation of theuniversal dimmer circuit;

FIG. 3—shows a universal dimmer circuit according to another aspect ofthe present invention, in which the load type is detected using aninductive load detector;

FIG. 4—shows a block diagram showing the use of a ringing detector in anaspect of the present invention;

FIG. 5—shows a voltage waveform across the dimmer controlling aresistive load;

FIG. 6A—shows a voltage waveform across a dimmer controlling aninductive load;

FIG. 6B—shows an expanded version of the waveform of FIG. 6A;

FIG. 7—shows a block diagram showing the main components of the ringingdetector of FIG. 4;

FIG. 8—shows the waveforms of the input and output of the peak detectorof FIG. 7;

FIG. 9—shows a block diagram showing the use of a voltage spike detectorin an aspect of the present invention;

FIG. 10—shows a circuit diagram of one form of the present invention.

DETAILED DESCRIPTION

According to an aspect of the present invention, the universal dimmer ofthe present invention employs a method which selects the appropriatedimmer operating mode each time the lamp load is activated.Consequently, at load switch off, the operating mode does not require tobe retained for the next load operation.

According to another aspect of the invention, one method of load typedetection involves the detection of the presence of dimmer voltageringing in response to a connected inductive load. Standard constructionlow voltage lighting transformers exhibit sufficient “leakageinductance” to produce the required dimmer voltage ringing behavior.Detection of dimmer voltage ringing while initially at low trailing edgemode conduction angles allows changeover to leading edge mode to occurwhile the lamp is at relatively low brightness levels—where any stepchange in effective applied load power is not noticeable (a lightingtransformer driven in trailing edge mode will produce a higher outputpower level than when driven in leading edge mode—for the same dimmerconduction angle setting. This difference is greater at higherconduction angles).

According to yet another aspect of the present invention, a secondarymethod of load type detection, acting in tandem to the principal methoddescribed above, allows for highly inductive loads such as fan motors toalso be readily detected, with resultant changeover to leading edgemode. For this method of detection, the trailing edge dimmer voltagespiking resulting from the highly inductive load is clamped to safeacceptable levels, whilst being monitored to instigate changeover toleading edge mode. In a similar fashion to that described above,detection of dimmer voltage spiking while initially at low trailing edgemode conduction angles allows changeover to leading edge mode to occurwhile the motor is at relatively low conduction angles ie. before themotor even begins to spin.

Only the relevant circuit sections for providing the discussedadditional function of automatic detection of connected inductive loadswill be described. Circuitry pertaining to the derivation of thenecessary drive control signals suitable for implementation of eitherreverse or forward phase control mode of dimmer operation is omitted, asthis may be standard circuitry in current universal dimmers and will bewell known to the person skilled in the art.

It is assumed that such a control circuit initially operates in reversephase, or trailing edge control mode, but is capable of being triggeredinto forward phase control when necessary.

Referring now to FIG. 2, there is shown a universal dimmer 1 connectedto and controlling power applied to load 30. Inside universal dimmer 1is a load type detector 2, which according to one aspect of theinvention, detects the type of the load 30 upon each activation of theuniversal dimmer 1.

In one specific form of the invention, load type detector 2 is providedby an inductive load detector 3, as shown in FIG. 3. In this aspect,universal dimmer 1 also has a mode control circuit 4, for changing themode of operation of universal dimmer 1. In this aspect, universaldimmer 1 begins its operation in the trailing edge mode, and upondetection of an inductive load by inductive load detector 3, which thengenerates a signal to mode control circuit 4, mode control circuit 4changes the operating mode of universal dimmer 1 from trailing edge toleading edge, as is known to the person skilled in the art.

Referring now to FIG. 4, there is shown a block diagram of one specificform of inductive load detector 3. Shown there is ringing detector 10,which receives as an input, a measure of the load voltage appearingacross the load 30. If load 30 is an inductive load, the use of trailingedge dimming will result in a voltage ringing signal induced across theload. Upon detection of this voltage ringing signal, ringing detector 10will generate a signal indicating the presence of an inductive load.

When used in a universal dimmer, this signal can be applied to knowncircuitry (not shown) to change the mode of operation of the universaldimmer from trailing edge to leading edge as will be understood by theperson skilled in the art.

Indeed, if the ringing circuit is used in another type of device that isrequired to detect ringing or to detect a load type, then this signalcould be used as required by that device. For example, if the device isa non-universal dimmer and can only operate in the trailing edge mode,then this signal may be used as a shutdown or other warning signal toprevent damage to the device and surrounding equipment.

In this particular embodiment, the indicating signal is provided to themode control circuitry via a latch 20.

To facilitate understanding of the function of the arrangement discussedabove and further below, reference will now be made to FIGS. 5, 6A and6B, which show waveforms of signals at various points in thearrangement.

FIG. 5 depicts the voltage across the dimmer at start-up at lowconduction angle in trailing edge mode, when connected to a resistiveload 30. As can be seen, at the end of each half-cycle conductionperiod, the dimmer voltage rises from near zero volts to instantaneousline voltage.

FIG. 6A now depicts the voltage across the dimmer at start-up at lowconduction angle in trailing edge mode, when the dimmer 1 is connectedto an inductive load. In this case, at end of the half-cycle conductionperiod, the dimmer voltage rises from near zero volts to momentarilyexceed the instantaneous line voltage.

FIG. 6B shows an expanded waveform of FIG. 6A, depicting voltage acrossthe dimmer 1 at start-up at low conduction angle, when connected to aninductive load.

The voltage oscillation or ringing occurs at a frequency significantlygreater than the line voltage frequency, as will be discussed furtherbelow, with ringing amplitude Vpk.

Turning now to FIG. 7, there is shown the main elements of ringingdetector 10. At the input of ringing detector 10 is a filter 11 forextracting the high frequency component described above with referenceto FIG. 6B. This signal component is generally about 1 kHz ringingsignal from the mains ac voltage (typically 50 Hz-60 Hz depending uponthe country and application) applied to the load.

This extracted signal is then applied to peak detector 12, which detectsthe peaks in the ringing signal. The detection of these peaks thenproduces a signal which is input to comparator 13. A reference voltageis also applied to a second input of comparator 13. When the level ofthe output signal from peak detector 12 exceeds the reference voltage,comparator 13 will generate a signal indicating the presence of ringing,which in turn indicates that load 30 is an inductive load.

FIG. 8 shows the output waveform from filter 11 shown in FIG. 7, whichis a high pass filter to reject line voltage frequency component, thusonly the ringing waveform component of amplitude Vpk appears at theinput to the peak detector 12.

The peak detector has an effective response time equal to several cyclesof line voltage period, therefore the peak output voltage is achievedonly after a corresponding number of consecutive ringing waveform eventshave occurred.

When the magnitude of the peak detector output exceeds the associatedreference voltage, the following comparator activates a latch circuit tochange the dimmer operation to leading edge mode. FIG. 8 shows thewaveform of the output of the peak detector rising to a value of Vpk.FIG. 8 also shows the value of Vref in dotted lines for reference.

When the dimmer operation changes to leading edge mode the peak detectoroutput slowly falls to zero, due to relatively slow decay time constant.

It will be understood that when this arrangement is used in a universaldimming circuit, this output may be used to trigger a known mode controlcircuitry to change the mode of operation of the universal dimmingcircuit from trailing edge to leading edge as described above.

FIG. 9 shows FIG. 4 with an additional spike detector 40. Spike detector40 may be used in conjunction with ringing detector 10 to furtherenhance the performance of the arrangement of the present invention.

The circuit and the operation of the preferred embodiment of the presentinvention will now be described in detail with reference to FIG. 10.

The circuit of the present invention can be divided into severalfunctional blocks as follows:

Power Transistor Drive Circuit

The load conducting elements in a typical reverse phase control dimmercomprise a pair of transistors such as MOSFET devices. Suitable gateinput drive circuitry is required to provide control of switchingtransition time, as a means of limiting EMI (ElectromagneticInterference) emission levels (strict Standards apply in the industry tolimit the level of EMI coming from devices such as dimmers).

Transistors Q9 & Q10 as seen in FIG. 5 connect in series back-to-backfashion to form an ac switch for control of the connected load. In thisembodiment, transistors Q9 and Q10 are MOSFETs (Metal-OxideSemiconductor Field Effect Transistors), often used in power controlapplications. Resistors R23 & R24 assist in preventing parasiticoscillation that can occur with parallel connected MOSFETs.

Application of drive voltage to resistor R16 results in conduction oftransistor Q6, which simultaneously causes transistor Q8 to be cutoffand transistor Q7 to also conduct and therefore result in activation ofthe load controlling ac switch.

The values of resistors R18, R19 & R20 are selected to ensure that Q7conduction status follows that of Q6, while Q8 assumes the oppositeconduction status. Q7 provides a level shifting function, while Q8provides an inversion function.

Resistor R21 limits turn-on current via Q7 to ac switch gate input,necessary to achieve controlled leading edge switching transition times,particularly when the dimmer operation is in the forward phase orleading edge control mode.

Resistor R22 limits turn-off current via Q8 from ac switch gate input,necessary to achieve controlled trailing edge switching transitiontimes, particularly when dimmer operation is in reverse phase controlmode.

Level shifting transistor Q5, with bias resistors R13, R14 & R15 andblocking diode D6, is arranged to have conduction status opposite tothat of Q6. This transistor provides an inverted pull-up drive formomentary enabling of the ringing detector circuit, which is describedin more detail below.

Ringing Detector Circuit

Capacitor C1 determines, in part, the ringing signal amplitude andfrequency at each mains half-cycle dimmer turn-off transition. Diodes D1& D2, in conjunction with intrinsic inverse diodes associated with Q9 &Q10, form a diode bridge to provide full-wave rectification of dimmervoltage waveform. At each mains half-cycle turn-off transition aninitially rising voltage, followed by a ringing voltage component,centered around the instantaneous mains voltage, appears at the dc sideof the bridge.

At the rising dimmer voltage transition, input coupling capacitor C3becomes charged, via series components resistor R9, diode D4 & 15V dcrail, to a level equal to the peak ringing voltage.

During the first ringing cycle, where the voltage is then falling, diodeD4 becomes reverse biased while D5 is forward biased. This allowspartial charge storage transfer from C3, via resistors R9, R26 & diodeD5, into detector output capacitor C4—which consequently develops anegative voltage with respect to the 15V rail.

The charge delivered to detector output capacitor C4 accumulates witheach mains half-cycle ringing event, with corresponding increase incapacitor voltage.

Filter input resistor R26 provides a high frequency “noise” rejectionfunction. Resistor R27 represents loading on the dc side of the diodebridge circuit, necessary to ensure the bridge output voltage can fallat a similar rate to that of the dimmer terminal voltage. Such loadingelements can be provided by current source elements for 15V rail.

Detector discharge resistor R10 has sufficient value that the rate ofdischarge of detector output capacitor C4 is relatively slow compared tothe charging pulse repetition rate.

Diode D5 prevents discharge of detector output capacitor C4 during therising dimmer voltage transitions. This is necessary for the “detection”function.

Resistor R25 in conjunction with input coupling capacitor C3, provides ahigh-pass-filter function to prevent the relatively slow falling rate ofchange of mains instantaneous voltage from contributing to the detectoroutput voltage. This provides the function of block 11 in FIG. 7.

The comparator circuit 13 (see FIG. 7) comprises bias current resistorR8, reference zener diode Z2 and transistor Q3. The emitter terminal ofQ3 forms the comparator input, while the collector terminal forms theoutput.

The latch circuit 20 comprises transistors Q1 & Q2 and bias currentresistors R4, R5, R6 & R7. The base terminal of Q1 forms the latch inputwhich is driven from the comparator 13 output, while the collectorterminal forms the output.

Detector disable transistor Q4, is normally biased into conductionthrough base current supply resistor R11. Under such conditions thecharge source for detector output capacitor C4 is shunted thus disablingthe ringing detector. Transistor Q5, in conjunction with capacitor C5and resistor R11 is used to momentarily remove bias supply to Q4 at eachmains half-cycle turn-off transition, thus enabling the ringingdetector. This is done to minimize the susceptibility of the detector tosurrounding electrical noise as previously described.

Q5 is biased by resistors R13, R14 & R15 when Q7 is not in theconducting state. In the Q7 conduction state diode D6 acts to removebase current supply for Q5.

Over-Voltage Detection Circuit

As described above, an additional function which may be implemented toenhance the function of the circuit is the use of an over-voltage, orvoltage spike, detection circuit.

Dimmer operation in reverse phase control mode, when connected to highlyinductive loads such as iron core transformer based neon lighting, willresult in excessive voltage spiking across dimmer terminals at eachmains half-cycle dimmer turn-off transition.

Turning off the switch while there is any appreciable level of currentcauses a sudden rise in the voltage appearing across the load. Asdescribed by the well known relationship

V=L*dI/dt

Where V is the voltage appearing across the inductive load;

-   -   L is the magnitude of the inductance of the load; and    -   dI/dt is the rate of change of the current I through the load        over time t        as will be understood by the person skilled in the art.

As can be seen, the greater the rate of change in current I through theload, the greater the voltage spike occurring. It follows then that thegreater the current at the time of turning off the switch, which causesthe current to fall to zero in a very short space of time, the greaterthe rate of change of current and therefore the greater the voltagespike induced.

The over-voltage detection circuit arrangement functions firstly tosafely limit spiking voltage amplitude, then activate a latch circuitafter spiking has been detected for a number of successive mains voltagehalf-cycles (for example, 3-6 half cycles).

Once triggered, the latch output status is used to signal to the dimmercontrol circuit mechanism that dimming operation should be changed toforward phase, or leading edge, control mode.

With reference again to FIG. 10, diodes D1 & D2, in conjunction withintrinsic inverse diodes associated with Q9 & Q10, form a diode bridgeto provide full-wave rectification of dimmer voltage waveform asdescribed above in relation to the ringing detector. The series circuitof varistor MV1 and zener diode Z1 provide the necessary dimmer terminalvoltage spiking clamping function.

During clamping events, a voltage is developed across resistor R1 anddetector output capacitor C2 is able to be charged via resistor R2 anddiode D3. Blocking diode D3 prevents discharge of C2 via R2 and R1during the relatively long time interval between voltage spiking events.

Filter input resistor R2 provides high frequency “noise” rejectionfunction. Output resistor R3, in conjunction with existing latch circuitinput resistor R5 and transistor Q2, are used to form a rudimentarycomparator function to determine the required detector output voltagelevel to trigger the latch.

It will be appreciated that the above has been described with referenceto a particular embodiment however, many variations and modificationsmay be made within the scope of the present invention.

In particular, the circuit may be used as a voltage ringing detector foruse in any suitable application. Furthermore, while the invention hasbeen described in the context of a universal dimmer to enable it toautomatically detect the type of load that is connected to the dimmer,it will be appreciated that the circuit may be applied to a conventionaluniversal dimmer which will determine its operating mode upon firstconnection to a load and retain that operating information in memory forits continued use in that mode.

Alternatively, one or more of the circuits described may even be able tobe used in a non-universal dimmer as a safety precaution to shut down atrailing edge dimmer that is accidentally connected to an inductiveload.

It will also be understood that throughout this specification, unlessthe context requires otherwise, the words ‘comprise’ and ‘include’ andvariations such as ‘comprising’ and ‘including’ will be understood toimply the inclusion of a stated integer or group of integers but not theexclusion of any other integer or group of integers.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgement or any form of suggestion that suchprior art forms part of the common general knowledge.

1. A universal dimmer for controlling power applied to a load, theuniversal dimmer comprising a load type detector which in use, detects aload type upon each activation of the universal dimmer.
 2. A universaldimmer as claimed in claim 1 wherein the universal dimmer is adapted tooperate in the trailing edge mode upon activation and the load typedetector comprises an inductive load detector for detecting the presenceof an inductive load, and the universal dimmer further comprises a modecontrol circuit to change the operation of the universal dimmer toleading edge mode if an inductive load is detected by the inductive loaddetector.
 3. A universal dimmer as claimed in claim 2 wherein theinductive load detector comprises: a voltage ringing detector fordetecting a voltage ringing signal across the load; and a signalgenerator for generating a signal indicating the presence of aninductive load upon the voltage ringing detector detecting said voltageringing signal.
 4. A universal dimmer as claimed in claim 3 wherein thevoltage ringing detector comprises a peak detector for detecting a peakvalue of the voltage ringing signal and for generating a dc signalcorresponding to the detected peak value.
 5. A universal dimmer asclaimed in claim 4 wherein the inductive load detector further comprisesa dc accumulator for accumulating the dc signal over time to provide anaccumulated dc signal.
 6. A universal dimmer as claimed in claim 5wherein the inductive load detector further comprises a comparator forcomparing the accumulated dc signal with a reference voltage andproducing an output when the accumulated dc signal exceeds the referencevoltage.
 7. A universal dimmer wherein the inductive load detectorfurther comprises an inductive load indicator for generating a signalindicating the presence of the inductive load when the output of thecomparator indicates the accumulated dc signal exceeds the referencevoltage.
 8. A universal dimmer as claimed in claim 7 wherein theinductive load detector further comprises a voltage spike detector fordetecting the presence of a voltage spike across the load and forgenerating a signal indicating the presence of the voltage spike.
 9. Auniversal dimmer as claimed in claim 3 wherein the mode control circuit,in use, changes the mode of operation of the dimmer circuit fromtrailing edge to leading edge operation upon detection of the voltageringing signal.
 10. A universal dimmer as claimed in claim 8 wherein themode changing circuit, in use, changes the mode of operation of thedimmer circuit from trailing edge to leading edge operation upondetection of the voltage spike.
 11. A method of operating a universaldimmer used to control power applied to a load, the method comprisingcausing the universal dimmer to detect a type of the load to which it isconnected upon each activation of the universal dimmer circuit.
 12. Amethod as claimed in claim 11 further comprising causing the universaldimmer to operate in the trailing edge mode upon activation and tochange to the leading edge mode if an inductive load is detected.
 13. Amethod as claimed in claim 12 wherein the step of detecting the presenceof the inductive load comprises: detecting a voltage ringing signalacross the load; and generating a signal indicating the presence of aninductive load upon detecting said voltage ringing signal.
 14. A methodas claimed in claim 13 further comprising detecting a peak value of thevoltage ringing signal and generating a dc signal corresponding to thedetected peak value.
 15. A method as claimed in claim 14 furthercomprising only detecting the peak value of the voltage ringing signalduring a brief period at each half cycle of the voltage signal appliedto the load so as to minimise the effect of electrical noise.
 16. Amethod as claimed in claim 14 further comprising accumulating the dcsignal over time to provide an accumulated dc signal.
 17. A method asclaimed in claim 16 further comprising comparing the accumulated dcsignal with a reference voltage and producing an output when theaccumulated dc signal exceeds the reference voltage.
 18. A method asclaimed in claim 16 further comprising generating a signal indicatingthe presence of the inductive load when the accumulated dc signalexceeds the reference voltage.
 19. A method as claimed in claim 12further comprising detecting the presence of a voltage spike across theload and generating a signal indicating the presence of the voltagespike.
 20. A method as claimed in claim 13 further comprising changing amode of operation of a dimmer circuit from trailing edge to leading edgeoperation upon detection of the voltage ringing signal.
 21. A method asclaimed in claim 19 further comprising changing a mode of operation of adimmer circuit from trailing edge to leading edge operation upondetection of the voltage spike.